Electrically drivable light modulators may be required for correcting the phases of light in a great variety of optical devices. Especially in the field of adaptive optics, efforts are intensifying to find ways to correct local unsharpness in the object image of a telescope or a camera, caused, for example, by atmospheric effects or thermal stresses in the equipment. Especially in connection with rapid digital image processing technology, ways are evolving to correct images that are so distorted. The image is corrected, even during observation, by an electronically drivable raster of optically active elements that is introduced into the optical path of rays of the object image. This leads one to imagine photographic and observational equipment, which delivers a sharp image even in the presence of strong disturbances.
Due to their electro-optical properties, liquid crystals can be used to control the phase of a light wave propagating through them, in that the refractive index of a layer of liquid crystals is influenced by an electric field. The principal electro-optical effects in liquid crystals alter both the birefringence as well as the orientation of the indicatrix of the refractive index of the liquid crystal. This is undesirable for most applications, because not only does the anisotropic character of the liquid crystals produce a phase shift as it passes through the liquid crystal, but it changes the polarization as well. For that reason, only polarized light can be handled using liquid crystals of this kind. However, wavefront variations caused by phase lag ought to be possible, independently of polarization, for the above mentioned applications.
When nematic liquid crystals are used, the phase of a light beam can be changed without affecting the polarization of the light. Here as well, however, the light must be linearly polarized in parallel to the director of the oriented liquid crystal molecules. In addition, the reaction rate of such cells is too slow for applications in image-processing devices.
Ferroelectric liquid crystals (FLC) can be driven in a sufficiently short operation time. However, potential applications for ordinary ferroelectric liquid crystals are very limited due to the small phase changes that are attainable with. At cell thicknesses of 10 μm, phase shifts of merely about {fraction (1/10)} of the wavelength of visible light are attained. However, one should aspire to phase shifts of one complete wavelength or more in order to perform all necessary phase corrections.
European Patent Application No. 0 309 774 discusses a liquid crystal cell which employs the DHF effect (deformation of the helix structure in the electric field) that occurs in FLCs, for continuous phase control and for gray scale representation. The optical phase control is based on a pronounced change in the average refractive index of the liquid crystal resulting from an applied electric field. The change in the birefringence of the deformed helix structure can reach d(n) =5%; the average refractive anisotropy <dn>=15%. Due to the optical properties of the helical structure of thereby employed chiral smectic liquid crystal in the electric field, the change in the birefringence is associated with marked changes in the orientation of the average optical indicatrix. This means that, after propagating through the liquid crystal, the light is in a polarization state which is heavily dependent upon the polarization state upon entry. This dependence precludes the use of the described cell for the required purposes.
The reference of Love, Restaino, Carreras, Loos, Morrison, Baur and Kopp: “Polarization Insensitive 127-Segment Liquid Crystal Wavefront Corrector”, Adaptive Optics, vol. 13, pp. 228-290, Optical Society of America, Washington D.C., 1996, discusses an electro-optically functioning modulator for controlling the phase of unpolarized light is introduced that contains two liquid crystal layers of the nematic type, which are disposed one behind the other. As apparently discussed in the reference, the two nematic layers are situated such that the directors of the liquid crystals are disposed orthogonally to one another in the field-free state. However, as already mentioned above, the cell may be much too slow for the purpose aspired to.